The present invention relates to charged particle beam lithography and more particularly to immersion lenses for such lithography.
Lithography is a technique used to fabricate semiconductor devices and integrated circuits. In lithography, a target substrate (usually a mask blank or semiconductor wafer) is coated with one or more layers of photoresist materials (resist). The resist is selectively exposed to a form of radiation, such as ultraviolet light, x-rays, electrons, and ions. The resist is then developed to remove part of the resist. The remaining part of the resist protects the underlying regions of the target. Regions from which the resist has been removed are subject to various additive (e.g., lift-off) or subtractive (e.g., etching) processes that transfer a pattern onto the target surface.
An electron beam or ion beam lithography system 110 (shown in FIG. 2) includes a charged particle (electron or ion) source 184 that generates a charged particle beam 116 directed through aperture plates 118, a blanking deflector 121, and focusing lenses 120 before reaching a final magnetic lens 112. Lens 112 further directs beam 116 onto a target 159 held on a target support 122 (also known as a stage). These lenses are electromagnetic or electrostatic, not light optic, structures. Charged particle source 184 generates the electron or ion beam. A control computer 123 controls the operation of lithography system 110.
One type of such lithography system is the variable axis immersion lens electron beam system, see, for example, U.S. Pat. No. 4,544,846, to Langner et al. (herein xe2x80x9cLangner et al.xe2x80x9d), incorporated by reference in its entirety. FIG. 2 and FIG. 4A of Langner et al. are reproduced respectively as FIG. 1A and FIG. 1B of the present disclosure. The Langner et al. Background section describes a variable axis electron beam projection system as being one where the electron optical axis of the projection system is shifted so as to be coincident with a deflected electron beam used to write on the target at all times. Shifting the electron optical axis is said to cause the electron beam to always land perpendicular to the target and to eliminate lens aberrations which are caused by off-axis electron beams.
The variable axis immersion lens electron beam system includes deflection coils 43 and 45 (see FIG. 1A that depicts this structure in cross-section) that deflect an electron beam (shift the axis of the electron beam) so as to direct the beam to the desired location on the target 59. An immersion lens 12 includes one or more excitation coils 41 and 53 that generate a magnetic field when conducting an electric current (also called the focusing magnetic field). The focusing magnetic field has magnetic field lines that extend from a pole piece 13 to a pole piece 14 (FIG. 1B). The focusing magnetic field thus immerses a target 59 in an approximately uniform magnetic field (hence the name immersion lens) where the magnetic field strength is maximum near the surface of pole piece 14.
A deflection coil 11 generates a magnetic field (also called the deflection magnetic field) that shifts the magnetic axis of immersion lens 12 (hence the name variable axis) to coincide with the shifted axis of the electron beam. Deflection coils 11, 43, 45 vary the deflection magnetic field over time as the axis of the electron beam is shifted to scan target 59 during lithographic processes.
The varying deflection magnetic field creates eddy currents in electrically conductive system components downstream from deflection coil 11 (with respect to the direction of propagation of the electron beam), such as a target (wafer or mask blank) holder 16, a holder handler 20, and pole piece 14. Additionally, the varying deflection magnetic field may create eddy currents in a target 59 that is, e.g., a semiconductor wafer. The eddy currents in the above-described elements generate opposing deflection fields that deflect the electron beam, thereby creating placement error of the electron beam.
Accordingly, a disadvantage of the variable axis immersion lens is placement error caused by eddy currents generated by the deflection magnetic field. Alternatively, the system components subject to the deflection magnetic field can be of non-electrically conductive materials. However, the cost of the system increases with use of such materials. Thus, what is needed is a method and an apparatus that prevent the deflection magnetic field from radiating into electrically conductive components of the system downstream from the deflection coil, without adversely affecting the focusing magnetic field.
In one embodiment, an immersion lens for a charged particle beam system includes a magnetically floating ferrite disk that shields non-magnetic but electrically conductive components of the system from the time varying magnetic field generated by the deflection coil while not disturbing the static magnetic field used for beam focusing. (Floating here means not forming a part of a magnetic circuit.) The disk is mounted downstream from the deflection coil (with respect to the direction of propagation of the charged particle beam) such that a surface of the disk is approximately parallel to a magnetic equipotential surface of the magnetic field (also called the focusing magnetic field) generated by the immersion lens. The disk limits the deflection magnetic field from radiating into the electrically conductive system components downstream from the disk.
In another embodiment, an immersion lens for a charged particle beam system includes a somewhat similar magnetically floating ferrite cone that shields electrically conductive elements from the deflection magnetic field. The cone is similarly mounted downstream from the deflection coil such that the surface of the cone is parallel or approximately parallel to a magnetic equipotential surface of the focusing magnetic field. The cone limits the deflection magnetic field from radiating into the electrically conductive system components downstream from the cone.
Various embodiments will be more fully understood in light of the following detailed description taken together with the accompanying drawings.